LCD Refresh rates vs response times

[AndrewAPrice]

If an LCD display has a response rate of between 2ms (best case) and 5ms (worst case), then due to simple math one would assume 1/0.005 = 200 refreshes per second. Yet why do these monitors only support a refresh rate of 75Hz? I've heard speculation that the limited bandwidth of DVI doesn't allow anything greater than 60-75Hz, yet that doesn't explain displays that still feature VGA input (through which I have achieved 200Hz on an old CRT).

If a monitor can only process 60-75 updates per second, then why are LCD manufactures aiming for low single digit response times when 13.3ms (75Hz) and 16.75ms (60Hz) would be sufficient?

[Solar]

Nonononono....

I'll oversimplify to give a clear picture (pun intended):

On a CRT monitor, an electron beam walking the screen triggers the glow of the individual pixels on its surface. This glow jumps to max when the beam hits the pixel, and fades rapidly afterwards. If the beam walks too slowly (low refresh rate), the glow-and-fade cycle becomes visible (flicker). The higher the refresh rate, the less flicker.

LCD displays are different: The pixels are triggered by a complex electric "matrix", and remain lit unless instructed to do otherwise. There is no fading, which means a LCD can get away with much lower refresh rates while still maintaining a completely flicker-free screen.

The response time of a LCD measures the time it takes a pixel to change its state. A large response time means that quick movements will result in afterpictures being "smeared" all over the screen. The faster the response time, the sharper the picture.

You see, response time has nothing to do with refresh rate at all...

[AndrewAPrice]

Sorry, I understood that the LCD technology doesn't deal with refresh rates. I'll repeat what I was saying in more detail.

From what I understand, the monitor still has a 'refresh rate'/frame rate, since the video card is steaming each frame one by one to the LCD display at a particular rate. This rate would be built into the processor inside of the monitor which inputs each frame and outputs the required state changes to the display matrix. Am I correct so far?

What I don't understand, is if it takes 5 milliseconds for a pixel to change from one state to another, then within a period of 1 second that pixel should be able to change to 200 different states. As long as the time between frames is greater than the time it takes for a pixel to change state then the image shouldn't trail behind/'ghost'. So why does the video processor inside the display artificially limit the monitor to handling 75 frames per second from the video card rather than limit it to the worst case response time (5ms = 200fps)?

[AJ]

Hi,

As far as I understand, the response time is the time taken for any single picture to turn from white to grey. There are no particular standards AFAIK, so manufacturers will tend to be a bit...um...optimistic. I think it's best to think of a response times as a "best case for certain state changes".

Cheers,
Adam

[Solar]

Not a specialist on LCD technology, so I won't judge if your understanding of LCD "frame rate" is correct or not. Sounds good to me, though.

What I don't understand, is if it takes 5 milliseconds for a pixel to change from one state to another, then within a period of 1 second that pixel should be able to change to 200 different states. As long as the time between frames is greater than the time it takes for a pixel to change state then the image shouldn't trail behind/'ghost'. So why does the video processor inside the display artificially limit the monitor to handling 75 frames per second from the video card rather than limit it to the worst case response time (5ms = 200fps)?

First, the response time given by the LCD manufacturer is an average, or more likely, a best-case value of the time a pixel takes to get to its "new" state. And if you switch the pixel again as soon as it reached its new state, you would never get a fully developed picture. Any refresh rate faster than, or even close to, the worst-case response time (black to white) would result in a smear in certain situations (high-contrast movements).

I agree that, in the meantime, response times have gotten to the point where faster refresh rates would be feasible. But look at it from the LCD manufacturer's standpoint: 60-75Hz LCD electronics have been a virtually bug-free, mass-produced item for a couple of years now, allowing them to compete on the price. And anyone except power gamers wouldn't even notice the difference between a 60 Hz and a, say, 120 Hz LCD, so why bother paying the R&D budget when response times of LCD crystals didn't allow for much better anyway until recently? Especially when the target audience (power gamers) actually stuck to their cheap-as-hell CRT's (which you couldn't beat for response time / refresh rate anyway)?

As far as I heard, they're catching on now, slowly producing faster (in terms of refresh rate) displays, so I guess in a couple of years the issue is moot.

[monkeykoder]

If I'm not mistaken anything faster than approx 30Hz is actually not generally noticeable as that is the approximate we'll use refresh rate of the human eye.

[xvedejas]

If I'm not mistaken anything faster than approx 30Hz is actually not generally noticeable as that is the approximate we'll use refresh rate of the human eye.

I've read that this value for the human eye is more nearly 60Hz, actually.

[monkeykoder]

From a quick google search I'm going to guess we're both wrongish. I still don't think that you need the faster refresh rate of a CRT though.

[NickJohnson]

But if the eye can only see things slower than 60Hz, why can I move my cursor up and down really fast and still see separate images instead of a blur? The point is, the refresh rate you need to make the picture not stagger depends on how fast the picture is moving. If you had a static picture, it would be impossible to tell whether your refresh rate is 1nHz or 1GHz. It's also fine with me if the refresh rate on an LCD means pixels don't have time to change completely before the next frame - a slight natural blur would probably make a better picture, and turn the stagger into something the human eye actually couldn't see.

Especially when the target audience (power gamers) actually stuck to their cheap-as-hell CRT's (which you couldn't beat for response time / refresh rate anyway)?

<hugs his 11-year-old Sony Trinitron running 1600x1200 at 75Hz>

[AndrewAPrice]

If I'm not mistaken anything faster than approx 30Hz is actually not generally noticeable as that is the approximate we'll use refresh rate of the human eye.

I have a LCD monitor with an advertised best-case 2ms refresh rate (the manual states worst-case is guaranteed to be 5ms) yet the refresh rate is locked at 75Hz. The reason I bring this up is because I was looking at stereoscopic shutter glasses and they recommend a refresh rate of at least 100Hz (50Hz per eye) to be bearable, 120Hz (60Hz per eye) to be comfortable. If the flickering is too noticeable it might be bad for my health since I'm epileptic.

I still have my old 19" CRT which can do 1024x768@150Hz/640x480@250Hz (goes right up to 2048x1536@60Hz) which I could use instead.

[Solar]

If I'm not mistaken anything faster than approx 30Hz is actually not generally noticeable as that is the approximate we'll use refresh rate of the human eye.

I've read that this value for the human eye is more nearly 60Hz, actually.

Let me guess... you're both some of the younger users on this forum, hm?

Did you ever work in front of a CRT?

60 Hz is a pain. While it's OK for a TV set (low resolution, comparatively low contrast), a computer display at 60 Hz flickers so much it gives you a headache, not even talking of PAL (50 Hz). 75 Hz is about the lowest bearable refresh rate, 85 Hz is somewhat of a standard, and they don't sell those 120 Hz "flicker-free" TV sets for nothing...

[xvedejas]

I've worked on a CRT for half of my life and 60Hz didn't seem bad to me. But then again, I don't exactly have the best eyes anyways

[Solar]

Well OK... I suffer from migraine attacks. Anything under 85 Hz and I don't see the end of the office day.

I remember spending some serious cash on those scandoubler / flicker fixer hardware on my Amigas...

[Love4Boobies]

60 Hz is a pain.

I actually think a lot of CRTs used to work at 60 Hz (without noticeable flickering).

[Solar]

And before there were dedicated CRT's, we hooked up home computers to TV sets.

We didn't realize how much they were flickering because, well, they all flickered, didn't they? Just like we did not realize how slow they were, because there was nothing faster available.

Try sitting down in front of a 7 MHz Amiga hooked up to an NTSC TV, and tell me it doesn't flicker (or crawl).